1. Field of the Invention
The present invention relates to an echo canceler to be interposed between a transmitter and a receiver of a bidirectional digital transmission device connected to a subscriber's line in an ISDN (integrated-services digital network) or the like, for generating a pseudoecho from a signal to be transmitted and an error signal to cancel an echo that is reflected from the transmitter toward the receiver.
2. Description of the Prior Art
Generally, echo cancelers are used to cancel echoes in bidirectional digital communications over subscribers' lines.
As shown in FIG. 1 of the accompanying drawings, a conventional echo canceler comprises a coder circuit 31, a driver circuit 32, a hybrid circuit 33, an echo canceling circuit, 34, and an adder 35.
A binary signal supplied to the echo canceler is applied to the coder circuit 31, which converts the supplied binary signal into a quaternary signal, or a 2B1Q signal, that comprises a 2-bit signal and represents any one of four states which the 2-bit signal can have. The converted quaternary signal is applied to the driver circuit 32 and the echo canceling circuit 34. The quaternary signal applied to the driver circuit 32 is transmitted through the hybrid circuit 33 to a subscriber's line 36. The transmitted signal received from the subscriber's line 36 is applied through the hybrid circuit 33 to the adder 35. The transmitted signal that has been received contains a quaternary signal (echo) which cannot be separated by the hybrid circuit 33.
The echo canceling circuit 34 is supplied with the transmitted signal and the quaternary signal, and generates a pseudoecho signal which is of the same waveform as and of opposite polarity to the echo contained in the transmitted signal. The pseudoecho signal is then applied to the adder 35 and added thereby to the transmitted signal. The sum signal is outputted from the adder 35 as a received signal.
The echo canceling circuit 34 comprises an adaptive digital filter having a tapped delay line for outputting the pseudoecho signal in response to the transmitted signal, a correlator for correlating the transmitted signal and the remaining echo signal contained in the received signal, and a multiplier for adjusting the tap coefficient of the tapped delay line based on an output signal from the correlator.
However, a conventional echo canceler has the disadvantage that if its operation clock signal experiences jitter, the pseudoecho signal and the actual echo will be displaced in timing from each other by an amount corresponding to the jitter and consequently, the echo canceler will fail to sufficiently cancel out the echo.